Basically, the interrogator of an IFF system requires an antenna with a directional pattern, which can be directed in any direction. Additionally, correct IFF ISLS (Identification Friend or Foe with Interrogation Sidelobe Suppression) and IFF RSLS (Identification Friend or Foe with Receiver Sidelobe Suppression) capability of an IFF interrogator requires that the antenna has an omnidirectional pattern, which transmits in and receives from all directions. An antenna with an omnidirectional pattern is easily achievable. However, the placement of antennas is a problem on naval ships. The only place where the omnidirectional pattern is preserved after placement on the ship is the top of the main mast. Lower placement means blocking structures are in the way of the antenna, which spoil the omnidirectional pattern. The top of the main mast is the place where all antennas with similar demands require to be located. Therefore a tradeoff needs to be done. Usually the IFF antenna is the one that looses against the antennas of the ESM (Electronic Support Measures) or the communications antennas.
The transponder of an IFF system also requires an antenna with an omnidirectional pattern, which transmits in and receives from all directions. Thus, the same placement problem arises. To tackle the coverage problem, a typical transponder has antenna diversity capability. This may allow usage of two antennas for example. The transponder replies to interrogations on the antenna with the strongest received interrogation. Then, it is a problem to place two antennas which together provide omnidirectional coverage. This is usually achieved by placing one antenna on starboard and one antenna on port.
The BAE OE-120/UPX is a cylindrical antenna array, which can be used for IFF interrogators. Indeed, such a cylindrical antenna array can be placed around the mast, thus not requiring the highest position, while maintaining an omnidirectional pattern for correct ISLS functioning. Despite IFF interrogators using a BAE OE-120/UPX antenna providing the IFF ISLS or RSLS capability, they have no capability to also act as the antenna for the IFF transponder. As a consequence, separate antennas for the transponder are still required, thereby giving rise to placement problems.